《航海学》课程参考文献（地文资料）CHAPTER 13 RADAR NAVIGATION

CHAPTER 13RADARNAVIGATIONPRINCIPLESOFRADARNAVIGATION1300.Introductiontopof the scope represents thedirection of the ship's headIn thisunstabilized presentation,the orientation changes asRadar determines distancetoan object by measuringthe ship changes heading. In the stabilized "north-upward"thetimerequiredforaradiosignal totravelfromatransmit-presentation,gyronorthisalwaysatthetopofthescope.ter to an object and return.Since most radars use directionalantennae, they can also determine an object's bearing1303.TheRadarBeamHowever,a radar's bearing measurement will be less accurate than itsdistance measurement.Understanding thisThepulsesofenergycomprising theradarbeamwouldconcept iscrucial to ensuringtheoptimal employmentofform a single lobe-shaped pattern of radiation if emitted inthe radar for safe navigation.free space.Figure 1303a.shows this free space radiationpattern, including the undesirable minor lobes or side lobes1301.SignalCharacteristicsassociatedwithpractical antennadesign.Inmostmarinenavigationapplications,theradar sigAlthough theradiatedenergy is concentrated intoarel-nal is pulse modulated. Signals are generated by a timingatively narrow main beam bythe antenna,there is nocircuitsothatenergyleavestheantennainveryshortpuls-clearly defined envelope of the energyradiated.The energyes.When transmitting,the antenna is connected to theis concentrated along the axis of the beam.With the rapidtransmitterbutnotthereceiver.Assoonasthepulseleavesdecrease in the amount of radiated energy in directionsanelectronic switchdisconnectstheantenna fromthetrans-awayfromthis axis,practical power limitsmaybeusedtomitter and connects it to the receiver.Another pulse is notdefine thedimensions of the radar beam.transmitted until after the preceding one has had time totravel to the most distant target within range and return.A radar beam's horizontal and vertical beam widths areSince the interval between pulses is long compared with thereferenced to arbitrarily selected power limits.The most com-length of a pulse, strong signals can beprovided with lowmon convention defines beam width as the angular widthaverage power.The duration or length of a single pulse isbetweenhalfpowerpoints.Thehalfpowerpointcorrespondscalled pulse length, pulse duration, or pulse width.Thisto a drop in3decibelsfromthemaximumbeam strength.pulse emission sequencerepeatsa greatmanytimes,per-The definition of the decibel shows this halving ofhaps 1,o00per second.This rate defines thepulsepower at a decrease in 3 dB from maximum power. A deci-repetition rate (PRR). The returned pulses are displayedbel is simply the logarithm of the ratio ofa final power levelonanindicatorscreen.to a reference power level:1302.The DisplayP1dB= 10 og[Po]Themost commontypeof radardisplayused intheNavy is the plan position indicator (PPID). On a PPL, thewhereP,isthefinal powerlevel,and Po isa reference pow-sweep starts atthecenter ofthedisplayand moves outwardalong a radial line rotating in synchronization with the an-erlevel.When calculating thedB dropfora50%reductiontenna.A detection isindicated by a brightening of theinpower level, the equation becomes:display screen at the bearing and range of the return.Be-dB = 10 log(.5)cause of a luminescent tube face coating,the glowcontinues after the trace rotates past the target.Figure 1302dB = -3 dBshowsthispresentationTheradiation diagram showninFigure1303bdepictsOn a PPI, a target's actual range is proportional to itsrelative values of power in the same plane existing at theecho'sdistancefromthescope's center.Amoveablecursorsame distances from theantenna or the origin of the radarhelps to measure ranges and bearings. In the "heading-up-beam.Maximum power is in thedirection oftheaxis ofthewardpresentation,which indicates relative bearings,the207

207 CHAPTER 13 RADAR NAVIGATION PRINCIPLES OF RADAR NAVIGATION 1300. Introduction Radar determines distance to an object by measuring the time required for a radio signal to travel from a transmit￾ter to an object and return. Since most radars use directional antennae, they can also determine an object’s bearing. However, a radar’s bearing measurement will be less accu￾rate than its distance measurement. Understanding this concept is crucial to ensuring the optimal employment of the radar for safe navigation. 1301. Signal Characteristics In most marine navigation applications, the radar sig￾nal is pulse modulated. Signals are generated by a timing circuit so that energy leaves the antenna in very short puls￾es. When transmitting, the antenna is connected to the transmitter but not the receiver. As soon as the pulse leaves, an electronic switch disconnects the antenna from the trans￾mitter and connects it to the receiver. Another pulse is not transmitted until after the preceding one has had time to travel to the most distant target within range and return. Since the interval between pulses is long compared with the length of a pulse, strong signals can be provided with low average power. The duration or length of a single pulse is called pulse length, pulse duration, or pulse width. This pulse emission sequence repeats a great many times, per￾haps 1,000 per second. This rate defines the pulse repetition rate (PRR). The returned pulses are displayed on an indicator screen. 1302. The Display The most common type of radar display used in the Navy is the plan position indicator (PPI). On a PPI, the sweep starts at the center of the display and moves outward along a radial line rotating in synchronization with the an￾tenna. A detection is indicated by a brightening of the display screen at the bearing and range of the return. Be￾cause of a luminescent tube face coating, the glow continues after the trace rotates past the target. Figure 1302 shows this presentation. On a PPI, a target’s actual range is proportional to its echo’s distance from the scope’s center. A moveable cursor helps to measure ranges and bearings. In the “heading-up￾ward” presentation, which indicates relative bearings, the top of the scope represents the direction of the ship’s head. In this unstabilized presentation, the orientation changes as the ship changes heading. In the stabilized “north-upward” presentation, gyro north is always at the top of the scope. 1303. The Radar Beam The pulses of energy comprising the radar beam would form a single lobe-shaped pattern of radiation if emitted in free space. Figure 1303a. shows this free space radiation pattern, including the undesirable minor lobes or side lobes associated with practical antenna design. Although the radiated energy is concentrated into a rel￾atively narrow main beam by the antenna, there is no clearly defined envelope of the energy radiated. The energy is concentrated along the axis of the beam. With the rapid decrease in the amount of radiated energy in directions away from this axis, practical power limits may be used to define the dimensions of the radar beam. A radar beam’s horizontal and vertical beam widths are referenced to arbitrarily selected power limits. The most com￾mon convention defines beam width as the angular width between half power points. The half power point corresponds to a drop in 3 decibels from the maximum beam strength. The definition of the decibel shows this halving of power at a decrease in 3 dB from maximum power. A deci￾bel is simply the logarithm of the ratio of a final power level to a reference power level: where P1 is the final power level, and P0 is a reference pow￾er level. When calculating the dB drop for a 50% reduction in power level, the equation becomes: The radiation diagram shown in Figure 1303b depicts relative values of power in the same plane existing at the same distances from the antenna or the origin of the radar beam. Maximum power is in the direction of the axis of the dB 10 P1 P0 = log - dB 10 .5·· ( ) ·· dB – 3 dB = = log

208RADARNAVIGATIONdimensionsoftheantennaFor agivenantenna size(antennaaperture),narrowerbeam widths resultfrom using shorter wavelengths.ForaNgiven wavelength,narrower beam widths result from usinglargerantennas.With radar waves being propagated in the vicinity ofthe surface of the sea, the main lobe of the radar beam iscomposed ofa numberof separatelobes,as opposedtothesingle lobe-shaped pattern of radiation as emitted in freeroSakspace.This phenomenon is the result of interferencebe-tweenradarwavesdirectlytransmitted,andthosewaves.0which are reflected from the surface of the sea. RadarXwaves strike the surface of the sea, and the indirect waves02reflectoff the surface of the sea.SeeFigure 1303c.TheseZakreflectedwaves either constructivelyordestructively inter-Bakuchi Misferewiththedirectwavesdependinguponthewaves'phaserelationship.1304.DiffractionAnd AttenuationDiffraction is thebending ofa waveas it passes an ob-struction.Because ofdiffractionthere is some illuminationoftheregionbehindanobstructionortargetbytheradatbeam.Diffraction effects aregreater at the lowerfrequen-cies.Thus,theradar beam ofa lowerfrequency radar tendsto illuminatemore oftheshadowregionbehind an obstruc-tion than the beam ofa radar of higher frequency or shorterwavelengthAttenuation is the scattering and absorption of theen-ergy in the radar beam as it passes through the atmosphere.It causes a decrease in echo strength.Attenuation is greaterat the higher frequencies or shorter wavelengthsWhilereflectedechoes aremuchweaker than thetrans-mitted pulses, the characteristics of their return to thesource are similarto thecharacteristics of propagation.Thestrengths oftheseechoes aredependentupontheamountoftransmitted energy striking thetargets and the size and re-flectingpropertiesofthetargets1305.RefractionHONSHU,NWCOAST-APPROACHIf the radar waves traveled in straight lines, the dis-TOMAIZURUKOtance to the radar horizon would be dependent only on theRadarposition:35°34.1N.,135°19.6E.poweroutput of thetransmitterand the height of the anten-Rangering interval:1milena. In other words, the distance to theradar horizon would(Source:JapaneseSailingDirections)bethe sameas thatofthegeometrical horizonfor theanten-naheight.However,atmospheric densitygradients bendradar rays as they travel to and from a target. This bendingis called refraction.Figure 1302. Plan Position Indicator (PPI) display.Thefollowing formula,where h is theheight of the an-tenna infeet,gives thedistancetotheradar horizon inbeam,Powervaluesdiminishrapidlyindirectionsawayfromnautical miles:the axis.The beam width is taken as the angle between the half-power points.d = 1.22 /hThebeam widthdepends upon thefrequency or wave-The distance to the radar horizon does not limit the dis-length of the transmitted energy, antenna design, and the

208 RADAR NAVIGATION beam. Power values diminish rapidly in directions away from the axis. The beam width is taken as the angle between the half￾power points. The beam width depends upon the frequency or wave￾length of the transmitted energy, antenna design, and the dimensions of the antenna. For a given antenna size (antenna aperture), narrower beam widths result from using shorter wavelengths. For a given wavelength, narrower beam widths result from using larger antennas. With radar waves being propagated in the vicinity of the surface of the sea, the main lobe of the radar beam is composed of a number of separate lobes, as opposed to the single lobe-shaped pattern of radiation as emitted in free space. This phenomenon is the result of interference be￾tween radar waves directly transmitted, and those waves which are reflected from the surface of the sea. Radar waves strike the surface of the sea, and the indirect waves reflect off the surface of the sea. See Figure 1303c. These reflected waves either constructively or destructively inter￾fere with the direct waves depending upon the waves’ phase relationship. 1304. Diffraction And Attenuation Diffraction is the bending of a wave as it passes an ob￾struction. Because of diffraction there is some illumination of the region behind an obstruction or target by the radar beam. Diffraction effects are greater at the lower frequen￾cies. Thus, the radar beam of a lower frequency radar tends to illuminate more of the shadow region behind an obstruc￾tion than the beam of a radar of higher frequency or shorter wavelength. Attenuation is the scattering and absorption of the en￾ergy in the radar beam as it passes through the atmosphere. It causes a decrease in echo strength. Attenuation is greater at the higher frequencies or shorter wavelengths. While reflected echoes are much weaker than the trans￾mitted pulses, the characteristics of their return to the source are similar to the characteristics of propagation. The strengths of these echoes are dependent upon the amount of transmitted energy striking the targets and the size and re￾flecting properties of the targets. 1305. Refraction If the radar waves traveled in straight lines, the dis￾tance to the radar horizon would be dependent only on the power output of the transmitter and the height of the anten￾na. In other words, the distance to the radar horizon would be the same as that of the geometrical horizon for the anten￾na height. However, atmospheric density gradients bend radar rays as they travel to and from a target. This bending is called refraction. The following formula, where h is the height of the an￾tenna in feet, gives the distance to the radar horizon in nautical miles: The distance to the radar horizon does not limit the dis￾Figure 1302. Plan Position Indicator (PPI) display. d 1.22 h =

209RADARNAVIGATIONFigure1303aFreespaceradiationpatternHalf-pouer poist(-3decitels)Bean widthleam asis.aHalf-power point(-3desbels)Figure1303b.Radiation diagram.Direct waveIndirwaveSea surfaceFigure1303c.Directand indirectwavestance from which echoes maybe received from targets.As-traveled an equal distance in the opposite direction.Atsumingthatadequatepoweristransmitted, echoes maybeB, thetransmitted pulsehas continued on beyond thereceived from targets beyond the radar horizon if their re-second target, and the two echoes are returning towardflecting surfaces extend above it. Note that the distance tothe transmitter.The distance between leading edges ofthe radar horizon is the distance at which the radar rays passthe two echoes is twice the distance between targets. Thetangent tothe surfaceof the earth.correct distance will be shown on the scope,which iscalibrated to show half the distance traveled out and1306.FactorsAffectingRadar Interpretationback. At C the targets are closer together and the pulselength has been increased.Thetwoechoesmerge, andRadar's value as a navigational aid depends on the nav-on the scope they will appear as a single, large target. Atigator's understanding its characteristics and limitations.D the pulse length has been decreased, and the two ech-Whether measuring the range to a single reflectiveobject oroesappear separated.The ability of aradarto separatetrying to discern a shoreline lost amid severeclutter,knowl-targets close together on the same bearing is called res-edge of the characteristics of the individual radar used areolution in range.It is related primarily to pulse length.crucial.Some of the factors to be considered in interpreta-The minimum distancebetweentargets that can bedis-tionarediscussedbelow:tinguished as separate is halfthe pulse length.This (halfthe pulse length) is the apparent depth or thickness of aResolution in Range. In part A ofFigure 1306a, a trans-target presenting a flat perpendicular surface to the radarmitted pulse has arrived at the second of two targets ofbeam.Thus, several ships close together mayappear asinsufficient size or density to absorb or reflect all of thean island. Echoes from a number of small boats, piles,energy of the pulse. While the pulse has traveled frombreakers, or even large ships close to the shore maythefirst to the second target, the echofromthefirst hasblend with echoes from the shore, resulting in an incor-

RADAR NAVIGATION 209 tance from which echoes may be received from targets. As￾suming that adequate power is transmitted, echoes may be received from targets beyond the radar horizon if their re￾flecting surfaces extend above it. Note that the distance to the radar horizon is the distance at which the radar rays pass tangent to the surface of the earth. 1306. Factors Affecting Radar Interpretation Radar’s value as a navigational aid depends on the nav￾igator’s understanding its characteristics and limitations. Whether measuring the range to a single reflective object or trying to discern a shoreline lost amid severe clutter, knowl￾edge of the characteristics of the individual radar used are crucial. Some of the factors to be considered in interpreta￾tion are discussed below: • Resolution in Range. In part A of Figure 1306a, a trans￾mitted pulse has arrived at the second of two targets of insufficient size or density to absorb or reflect all of the energy of the pulse. While the pulse has traveled from the first to the second target, the echo from the first has traveled an equal distance in the opposite direction. At B, the transmitted pulse has continued on beyond the second target, and the two echoes are returning toward the transmitter. The distance between leading edges of the two echoes is twice the distance between targets. The correct distance will be shown on the scope, which is calibrated to show half the distance traveled out and back. At C the targets are closer together and the pulse length has been increased. The two echoes merge, and on the scope they will appear as a single, large target. At D the pulse length has been decreased, and the two ech￾oes appear separated. The ability of a radar to separate targets close together on the same bearing is called res￾olution in range. It is related primarily to pulse length. The minimum distance between targets that can be dis￾tinguished as separate is half the pulse length. This (half the pulse length) is the apparent depth or thickness of a target presenting a flat perpendicular surface to the radar beam. Thus, several ships close together may appear as an island. Echoes from a number of small boats, piles, breakers, or even large ships close to the shore may blend with echoes from the shore, resulting in an incor￾Figure1303a. Freespace radiation pattern. Figure1303b. Radiation diagram. Figure1303c. Direct and indirect waves

210RADARNAVIGATIONrect indicationofthepositionandshapeoftheshorelinewavesmore strongly than a wooden surface.Asurfaceperpendicular to the beam returns a stronger echo thanResolution in Bearing.Echoes from two or more tar-a non perpendicular one.For this reason,a gently slop-gets close together at the same range may merge to formingbeachmaynotbevisible.Avessel encountereda single, wider echo.The ability to separate targets isbroadside returns a stronger echo than one heading di-called resolution in bearing.Bearing resolution is arectlytowardoraway.function of two variables:beam width and range be-tween targets. A narrower beam and a shorter distanceFrequency.As frequency increases, reflections occurbetween objects both increase bearing resolution.from smallertargets.Height of Antenna and Target. If the radar horizon isAtmospheric noise,sea return,and precipitation com-between thetransmitting vessel and thetarget,thelowerplicate radar interpretation by producing clutter.Clutter ispart of the target will not be visible. A large vessel mayusually strongest near thevessel.Strong echoes can some-appearasa small craft,orashorelinemayappearatsometimes be detected by reducing receiver gain to eliminatedistance inland.weaker signals.By watching the repeater during several ro-tations of the antenna, the operator can discriminateReflecting Quality and Aspect of Target. Echoesbetween clutterand a target even when the signal strengthsfrom several targets ofthe same sizemaybe quitedif-ferent in appearance.A metal surface reflects radiofrom clutter and the target are equal. At each rotation, theTARGETS0A41TRANSMITTED PULSETARGETSTRANSMITTEDPULSEECHOECHOB?TARGETSICECHOES4TRANSMITTED PULSETARGETSDECHOES/1TRANSMITTED PURSEFigure1306a.Resolution inrange

210 RADAR NAVIGATION rect indication of the position and shape of the shoreline. • Resolution in Bearing. Echoes from two or more tar￾gets close together at the same range may merge to form a single, wider echo. The ability to separate targets is called resolution in bearing. Bearing resolution is a function of two variables: beam width and range be￾tween targets. A narrower beam and a shorter distance between objects both increase bearing resolution. • Height of Antenna and Target. If the radar horizon is between the transmitting vessel and the target, the lower part of the target will not be visible. A large vessel may appear as a small craft, or a shoreline may appear at some distance inland. • Reflecting Quality and Aspect of Target. Echoes from several targets of the same size may be quite dif￾ferent in appearance. A metal surface reflects radio waves more strongly than a wooden surface. A surface perpendicular to the beam returns a stronger echo than a non perpendicular one. For this reason, a gently slop￾ing beach may not be visible. A vessel encountered broadside returns a stronger echo than one heading di￾rectly toward or away. • Frequency. As frequency increases, reflections occur from smaller targets. Atmospheric noise, sea return, and precipitation com￾plicate radar interpretation by producing clutter. Clutter is usually strongest near the vessel. Strong echoes can some￾times be detected by reducing receiver gain to eliminate weaker signals. By watching the repeater during several ro￾tations of the antenna, the operator can discriminate between clutter and a target even when the signal strengths from clutter and the target are equal. At each rotation, the Figure 1306a. Resolution in range

211RADAR NAVIGATIONsignals from targets will remain relatively stationary on thecause it lies too low in the water.display while those caused by clutter will appear at differ-Coral atolls and long chains of islands may produceentlocations.long linesof echoes when the radarbeam is directed per-Another major problem lies in determining which fea-pendicular to the line of the islands.This indication istures in the vicinity of the shoreline are actually representedespeciallytruewhen the islands areclosely spaced.The rea-by echoes shown on the repeater.Particularly in cases whereson is that the spreading resulting from the width of thea low lying shore is being scanned, there may be considerableradarbeamcauses theechoestoblend intocontinuous lines.uncertainty.Whenthechain of islands isviewed lengthwise,or oblique-ly,however,each island may produce a separatereturn.Arelated problem is that certainfeatures on the shoreSurf breaking on a reef around an atoll produces aragged,will not return echoes because theyare blocked fromthe ra-variablelineofechoesdar beamby other physical featuresor obstructions.Thisfactor in turn causes the chart like image painted on theOneortworocksprojectingabovethesurfaceof thescopetodifferfromthechart of thearea.water, or waves breaking over a reef, may appear on thePPL.Whenanobject is submergedentirelyandtheseaisIfthenavigator istobeableto interpretthepresentationsmooth over it, no indication is seen on the PPIonhisradarscope,hemustunderstandthecharacteristicsofradar propagation,the capabilities ofhisradar set,thereflectIf the land rises in a gradual, regular manner from theshoreline, no part of the terrain produces an echo that isingproperties of differenttypes of radartargets,and theability to analyze his chart to determine which charted fea-stronger than the echo from any other part. As a result, ageneral haze ofechoes appears on the PPI, and it is difficultturesaremostlikelytoreflectthetransmittedpulsesortobeblocked.Experiencegainedduringclearweathercomparisonto ascertain the range to anyparticularpart of the land.between radar and visual images is invaluable.Blotchy signals are returned from hilly ground, becauseLandmasses aregenerallyrecognizable because of thethe crest ofeach hill returns agood echo although the valleysteady brilliance of the relatively large areas painted on thebeyond is in a shadow.If high receiver gain is used, the pat-PPI.Also,land shouldbeatpositions expectedfromtheship'sternmaybecomesolidexceptfortheverydeep shadows.navigational position.AlthoughlandmassesarereadilyrecogLow islands ordinarily produce small echoes.Whennizable,theprimaryproblem is the identificationof specificthick palmtrees orotherfoliagegrowon the island,strongland features.Identification of specificfeatures canbequiteechoesoften areproducedbecausethehorizontal surfaceofdifficult because ofvariousfactors, including distortion resultthe water around the island forms a sort of corner reflectoringfrombeam widthandpulselength,anduncertainty astowiththevertical surfacesofthetrees.Asaresultwoodedjust which charted features are reflecting the echoes.islands give good echoes and can be detected at a muchSand spits and smooth,clearbeaches normallydo notgreater range thanbarren islands.appear on the PPI at ranges beyond 1 or 2 miles because theseSizable land masses may be missing from the radar dis-targets have almost no area that can reflectenergy backto theplaybecause ofcertainfeatures being blocked fromthe radarradar.Ranges determinedfrom these targets are not reliablebeam by other features.A shoreline which is continuous onIfwavesarebreakingoverasandbar,echoesmaybereturnedthePPI display when the ship is at one position, may not befrom the surf.Wavesmay,however,break well outfrom thecontinuous when the ship is at another position and scanningactual shoreline, so that ranging on the surf may bethe same shoreline.The radar beam may beblocked from amisleading.segment of this shoreline by an obstruction such as aprom-Mud flats and marshes normally reflect radar pulsesontory.An indentation intheshoreline,such as a coveorbayonlya littlebetterthan a sand spit.The weak echoes receivedappearing on the PPI when the ship is at one position, mayat lowtide disappearat high tide.Mangroves and other thicknotappear when theshipis atanotherposition nearby.Thusgrowth mayproduce a strong echo.Areas that areindicatedradar shadowalonecan cause considerabledifferencesbe-tween thePPI display and the chartpresentation.This effectasswampsonachart,therefore,mayreturn eitherstrongorweak echoes, depending on the density and size ofthe vegeinconjunction withbeamwidthandpulselengthdistortionoftation growing in the area.the PPI displaycan cause even greaterdifferences.When sand dunes are covered with vegetation and areThe returns of objects close to shore may merge withwellbackfromalow,smoothbeach,theapparentshorelinethe shoreline image on the PPI,because ofdistortion effectsdetermined by radarappears as the line of thedunes ratherof horizontal beam width and pulse length.Target imagesthan the true shoreline.Under some conditions,sand duneson thePPI always are distorted angularlybyan amountmayreturnstrongechosignalsbecausethecombinationofequal to the effectivehorizontal beam width.Also, thetar-the vertical surface of the vegetation and the horizontalget images always are distorted radiallyby an amountatbeachmayforma sortof cornerreflector.least equal to one-half the pulse length (164 yards per mi-Lagoons and inland lakes usuallyappearas blankareascrosecondofpulselength).onaPPI becausethesmoothwatersurfacereturnsnoenerFigure1306b illustratestheeffects of ship's position.beam width,and pulse length ontheradar shoreline.Be-gyto theradar antenna.In some instances,the sandbar orreef surrounding the lagoon may not appear on the PPI be-causeofbeam widthdistortion,a straight,ornearlystraight

RADAR NAVIGATION 211 signals from targets will remain relatively stationary on the display while those caused by clutter will appear at differ￾ent locations. Another major problem lies in determining which fea￾tures in the vicinity of the shoreline are actually represented by echoes shown on the repeater. Particularly in cases where a low lying shore is being scanned, there may be considerable uncertainty. A related problem is that certain features on the shore will not return echoes because they are blocked from the ra￾dar beam by other physical features or obstructions. This factor in turn causes the chart like image painted on the scope to differ from the chart of the area. If the navigator is to be able to interpret the presentation on his radarscope, he must understand the characteristics of radar propagation, the capabilities of his radar set, the reflect￾ing properties of different types of radar targets, and the ability to analyze his chart to determine which charted fea￾tures are most likely to reflect the transmitted pulses or to be blocked. Experience gained during clear weather comparison between radar and visual images is invaluable. Land masses are generally recognizable because of the steady brilliance of the relatively large areas painted on the PPI. Also, land should be at positions expected from the ship’s navigational position. Although land masses are readily recog￾nizable, the primary problem is the identification of specific land features. Identification of specific features can be quite difficult because of various factors, including distortion result￾ing from beam width and pulse length, and uncertainty as to just which charted features are reflecting the echoes. Sand spits and smooth, clear beaches normally do not appear on the PPI at ranges beyond 1 or 2 miles because these targets have almost no area that can reflect energy back to the radar. Ranges determined from these targets are not reliable. If waves are breaking over a sandbar, echoes may be returned from the surf. Waves may, however, break well out from the actual shoreline, so that ranging on the surf may be misleading. Mud flats and marshes normally reflect radar pulses only a little better than a sand spit. The weak echoes received at low tide disappear at high tide. Mangroves and other thick growth may produce a strong echo. Areas that are indicated as swamps on a chart, therefore, may return either strong or weak echoes, depending on the density and size of the vege￾tation growing in the area. When sand dunes are covered with vegetation and are well back from a low, smooth beach, the apparent shoreline determined by radar appears as the line of the dunes rather than the true shoreline. Under some conditions, sand dunes may return strong echo signals because the combination of the vertical surface of the vegetation and the horizontal beach may form a sort of corner reflector. Lagoons and inland lakes usually appear as blank areas on a PPI because the smooth water surface returns no ener￾gy to the radar antenna. In some instances, the sandbar or reef surrounding the lagoon may not appear on the PPI be￾cause it lies too low in the water. Coral atolls and long chains of islands may produce long lines of echoes when the radar beam is directed per￾pendicular to the line of the islands. This indication is especially true when the islands are closely spaced. The rea￾son is that the spreading resulting from the width of the radar beam causes the echoes to blend into continuous lines. When the chain of islands is viewed lengthwise, or oblique￾ly, however, each island may produce a separate return. Surf breaking on a reef around an atoll produces a ragged, variable line of echoes. One or two rocks projecting above the surface of the water, or waves breaking over a reef, may appear on the PPI. When an object is submerged entirely and the sea is smooth over it, no indication is seen on the PPI. If the land rises in a gradual, regular manner from the shoreline, no part of the terrain produces an echo that is stronger than the echo from any other part. As a result, a general haze of echoes appears on the PPI, and it is difficult to ascertain the range to any particular part of the land. Blotchy signals are returned from hilly ground, because the crest of each hill returns a good echo although the valley beyond is in a shadow. If high receiver gain is used, the pat￾tern may become solid except for the very deep shadows. Low islands ordinarily produce small echoes. When thick palm trees or other foliage grow on the island, strong echoes often are produced because the horizontal surface of the water around the island forms a sort of corner reflector with the vertical surfaces of the trees. As a result, wooded islands give good echoes and can be detected at a much greater range than barren islands. Sizable land masses may be missing from the radar dis￾play because of certain features being blocked from the radar beam by other features. A shoreline which is continuous on the PPI display when the ship is at one position, may not be continuous when the ship is at another position and scanning the same shoreline. The radar beam may be blocked from a segment of this shoreline by an obstruction such as a prom￾ontory. An indentation in the shoreline, such as a cove or bay, appearing on the PPI when the ship is at one position, may not appear when the ship is at another position nearby. Thus, radar shadow alone can cause considerable differences be￾tween the PPI display and the chart presentation. This effect in conjunction with beam width and pulse length distortion of the PPI display can cause even greater differences. The returns of objects close to shore may merge with the shoreline image on the PPI, because of distortion effects of horizontal beam width and pulse length. Target images on the PPI always are distorted angularly by an amount equal to the effective horizontal beam width. Also, the tar￾get images always are distorted radially by an amount at least equal to one-half the pulse length (164 yards per mi￾crosecond of pulse length). Figure 1306b illustrates the effects of ship’s position, beam width, and pulse length on the radar shoreline. Be￾cause of beam width distortion, a straight, or nearly straight

BYOURSHLANDLOWISLANDRADARSHORELINEYOURSHIPRADARSHORELINERADAR NAVIGATIONRADARSHORELINEYOURSHIPLANDLANDYOURSHILANDRADARSHORELNEYOURSHIPRADARSHORELINERADARSHORELINEANDLANDLANDYOURSHIPFigure 1306b.Effects of ship's position, beam width,and pulse length on radar shoreline

212 RADAR NAVIGATION Figure 1306b. Effects of ship’s position, beam width, and pulse length on radar shoreline

213RADARNAVIGATIONRadar ShadowTOSteep ShoreShip No,Ship NoBeam WidthLow SandBeachTowerTowerOnBeachDistortionB Your PositionShip No,2Ship No.Z+RadShadovBAFigure1306c.Distortion effectsofradarshadowbeamwidth,andpulselengthshorelineoftenappears crescent-shapedon thePPI.Thisef-1307.RecognitionOfUnwantedEchoesfect isgreater with the wider beam widths.Notethat thisdistortion increases astheanglebetweenthebeam axis andThenavigatormustbeabletorecognizevarious abnor-the shorelinedecreases.mal echoes and effects on the radarscope so as not to beFigure1306c illustrates thedistortion effects of radarconfused by their presence.shadow,beam width,andpulselength.ViewAshowstheIndirect orfalse echoes arecausedbyreflection of theactualshapeoftheshorelineandthelandbehindit.Notethemainlobeoftheradarbeamoffship'sstructuressuchassteeltower on thelow sand beach and thetwoships atan-stacks andkingposts.When suchreflection doesoccur,thechor close toshore.Theheavy line inviewBrepresents theechowillreturnfromalegitimateradar contacttotheanten-shoreline on the PPI.The dotted lines represent the actualna by the same indirect path.Consequently,the echo willposition and shape ofall targets.Note in particular:appear on the PPIat the bearingofthereflecting surface.Asshown in Figure 1307a, the indirect echo will appear on the1.The low sandbeach is not detected bytheradarPPIat the same range as the direct echo received, assuming2.Thetoweronthelowbeachisdetected,butitlookslikethat the additional distance by the indirect path isa ship in a cove. At closer range the land would be de-negligible.tected and the cove-shaped area would begin to fill in;then the tower could not be seen without reducing theCharacteristics by which indirectechoes may be recog-receivergainnized aresummarizedasfollows:3.Theradar shadow behind both mountains.Distortionowing to radar shadows is responsiblefor more confu-1. The indirect echoes will usually occur in shadowsion than any other cause. The small island does notsectors.appear because it is in the radar shadow.2Theyarereceived on substantially constant bear-4.The spreading of the land in bearing caused by beamings,although the truebearing oftheradar contactwidthdistortion.Look at theupper shore of thepeninsu-maychangeappreciably.3.la.The shorelinedistortion is greater to the west becauseTheyappear at the samerangesas thecorrespond-the anglebetween the radar beam and the shore is small-ing direct echoes.4When plotted, their movements are usuallyer as the beam seeks out the more westerly shore.Ship No.I appears as a small peninsula. Her return hasabnormal.5.merged with the land becauseof the beamwidth5.Their shapes may indicate that they are not directdistortion.echoes.6.ShipNo.2alsomerges withthe shorelineand formsaSide-lobe effects are readily recognized in that theybump.This bump is caused by pulse length andbeamwidth distortion.Reducing receiver gain might causeproduce a series of echoes (Figure 1307b) on each side ofthe ship to separatefromland,provided the ship is notthe main lobe echo at the samerange as the latter.Semicir-tooclosetotheshore.TheFastTimeConstant (FTC)cles,oreven completecircles,maybeproduced.Becauseofcontrolcouldalsobeusedtoattempttoseparatethe shipthe lowenergy ofthe side-lobes, these effects will normallyfrom land.occur only attheshorter ranges.Theeffectsmaybemini

RADAR NAVIGATION 213 shoreline often appears crescent-shaped on the PPI. This ef￾fect is greater with the wider beam widths. Note that this distortion increases as the angle between the beam axis and the shoreline decreases. Figure 1306c illustrates the distortion effects of radar shadow, beam width, and pulse length. View A shows the actual shape of the shoreline and the land behind it. Note the steel tower on the low sand beach and the two ships at an￾chor close to shore. The heavy line in view B represents the shoreline on the PPI. The dotted lines represent the actual position and shape of all targets. Note in particular: 1. The low sand beach is not detected by the radar. 2. The tower on the low beach is detected, but it looks like a ship in a cove. At closer range the land would be de￾tected and the cove-shaped area would begin to fill in; then the tower could not be seen without reducing the receiver gain. 3. The radar shadow behind both mountains. Distortion owing to radar shadows is responsible for more confu￾sion than any other cause. The small island does not appear because it is in the radar shadow. 4. The spreading of the land in bearing caused by beam width distortion. Look at the upper shore of the peninsu￾la. The shoreline distortion is greater to the west because the angle between the radar beam and the shore is small￾er as the beam seeks out the more westerly shore. 5. Ship No. 1 appears as a small peninsula. Her return has merged with the land because of the beam width distortion. 6. Ship No. 2 also merges with the shoreline and forms a bump. This bump is caused by pulse length and beam width distortion. Reducing receiver gain might cause the ship to separate from land, provided the ship is not too close to the shore. The Fast Time Constant (FTC) control could also be used to attempt to separate the ship from land. 1307. Recognition Of Unwanted Echoes The navigator must be able to recognize various abnor￾mal echoes and effects on the radarscope so as not to be confused by their presence. Indirect or false echoes are caused by reflection of the main lobe of the radar beam off ship’s structures such as stacks and kingposts. When such reflection does occur, the echo will return from a legitimate radar contact to the anten￾na by the same indirect path. Consequently, the echo will appear on the PPI at the bearing of the reflecting surface. As shown in Figure 1307a, the indirect echo will appear on the PPI at the same range as the direct echo received, assuming that the additional distance by the indirect path is negligible. Characteristics by which indirect echoes may be recog￾nized are summarized as follows: 1. The indirect echoes will usually occur in shadow sectors. 2. They are received on substantially constant bear￾ings, although the true bearing of the radar contact may change appreciably. 3. They appear at the same ranges as the correspond￾ing direct echoes. 4. When plotted, their movements are usually abnormal. 5. Their shapes may indicate that they are not direct echoes. Side-lobe effects are readily recognized in that they produce a series of echoes (Figure 1307b) on each side of the main lobe echo at the same range as the latter. Semicir￾cles, or even complete circles, may be produced. Because of the low energy of the side-lobes, these effects will normally occur only at the shorter ranges. The effects may be mini Figure 1306c. Distortion effects of radar shadow, beam width, and pulse length

214RADARNAVIGATION1NDirectecho1AntennaIndirectechoStackFigure1307a.Indirectecho.mized oreliminated, through use of the gain and anti-clutterElectronic interference effects, such as may occurcontrols. Slotted wave guide antennas have largely elimi-when near another radar operating in the same frequencynated the side-lobe problem.band as that of the observer's ship, is usually seen on thePPI as a large number of bright dots either scattered at ran-Multipleechoes may occurwhena strong echo is re-domorintheformofdotted linesextendingfromthecenterceived from another ship at close range.A second or thirdtotheedgeof thePPIormoreechoesmaybeobservedontheradarscopeatdou-ble,triple,orothermultiplesoftheactualrangeoftheradarInterference effects are greater at the longer radar rangecontact (Figure 1307c).scale settings.The interference effects can be distinguishedeasilyfrom normal echoes becausetheydo not appear intheSecond-trace echoes (multiple-trace echoes)are ech-same places on successive rotations ofthe antenna.oes received from a contact at an actual range greater thanStacks,masts,samsonposts,andotherstructures,maythe radarrange setting.If an echo from a distanttarget is re-ceived afterthefollowingpulsehas beentransmitted, thecause areduction in the intensity ofthe radar beam beyond theseechowillappearon theradarscopeatthecorrectbearingbutobstructions,especiallyiftheyareclosetotheradarantenna.Ifnot at the true range. Second-trace echoes are unusual, ex-the angle at the antenna subtended by the obstruction is morecept under abnormal atmospheric conditions, or conditionsthan a few degrees, the reduction of the intensity of the radarbeam beyond the obstruction may produce a blind sector.Lessunder which super-refraction ispresent.Second-trace ech-reduction in the intensityof thebeam beyond the obstructionsoesmayberecognized throughchanges intheirpositionsmayproduceshadowsectors.Withinashadowsector,small tar-on the radarscope in changingthe pulse repetition rategets at close range may not be detected, while larger targets at(PRR),theirhazy,streaky,ordistorted shape;andtheerrat-much greater ranges will appear.icmovements onplottingAs illustrated in Figure 1307d, a target return is detectedSpoking appears on the PPI as a number of spokes orradial lines. Spoking is easily distinguished from interfer-onatruebearingof090°atadistanceof7.5miles.Onchang-ence effects becausethe lines are straight on all range-scaleing the PRRfrom2,000 to 1,800 pulses per second, the sametargetis detectedon abearing of090°at adistanceof3milessettings, and are lines rather than a series of dots.The spokes may appear all around the PPL, or they may be(Figure 1307e).The change in the position of the return indi-cates that the return is a second-trace echo. The actualconfined to a sector.If spoking is confined to a narrow sector,distance of the target is the distance as indicated on thePPItheeffectcanbedistinguishedfromaRamarksignal of similarplus halfthe distance the radar wave travels between pulses.appearance throughobservation ofthe steady relative bearing

214 RADAR NAVIGATION mized or eliminated, through use of the gain and anti-clutter controls. Slotted wave guide antennas have largely elimi￾nated the side-lobe problem. Multiple echoes may occur when a strong echo is re￾ceived from another ship at close range. A second or third or more echoes may be observed on the radarscope at dou￾ble, triple, or other multiples of the actual range of the radar contact (Figure 1307c). Second-trace echoes (multiple-trace echoes) are ech￾oes received from a contact at an actual range greater than the radar range setting. If an echo from a distant target is re￾ceived after the following pulse has been transmitted, the echo will appear on the radarscope at the correct bearing but not at the true range. Second-trace echoes are unusual, ex￾cept under abnormal atmospheric conditions, or conditions under which super-refraction is present. Second-trace ech￾oes may be recognized through changes in their positions on the radarscope in changing the pulse repetition rate (PRR); their hazy, streaky, or distorted shape; and the errat￾ic movements on plotting. As illustrated in Figure 1307d, a target return is detected on a true bearing of 090° at a distance of 7.5 miles. On chang￾ing the PRR from 2,000 to 1,800 pulses per second, the same target is detected on a bearing of 090° at a distance of 3 miles (Figure 1307e). The change in the position of the return indi￾cates that the return is a second-trace echo. The actual distance of the target is the distance as indicated on the PPI plus half the distance the radar wave travels between pulses. Electronic interference effects, such as may occur when near another radar operating in the same frequency band as that of the observer’s ship, is usually seen on the PPI as a large number of bright dots either scattered at ran￾dom or in the form of dotted lines extending from the center to the edge of the PPI. Interference effects are greater at the longer radar range scale settings. The interference effects can be distinguished easily from normal echoes because they do not appear in the same places on successive rotations of the antenna. Stacks, masts, samson posts, and other structures, may cause a reduction in the intensity of the radar beam beyond these obstructions, especially if they are close to the radar antenna. If the angle at the antenna subtended by the obstruction is more than a few degrees, the reduction of the intensity of the radar beam beyond the obstruction may produce a blind sector. Less reduction in the intensity of the beam beyond the obstructions may produce shadow sectors. Within a shadow sector, small tar￾gets at close range may not be detected, while larger targets at much greater ranges will appear. Spoking appears on the PPI as a number of spokes or radial lines. Spoking is easily distinguished from interfer￾ence effects because the lines are straight on all range-scale settings, and are lines rather than a series of dots. The spokes may appear all around the PPI, or they may be confined to a sector. If spoking is confined to a narrow sector, the effect can be distinguished from a Ramark signal of similar appearance through observation of the steady relative bearing Figure 1307a. Indirect echo

215RADARNAVIGATIONFigure1307b.Side-lobeeffectsFigure1307c.MultipleechoesFigure 1307d. Second-trace echo on 12-mile range scale.Figure 1307e.Position of second-trace echo on 12-milerangescaleafterchangingPRR.of the spoke in a situation where thebearing of the Ramark sig-linear timebaseorthesweepnotstartingontheindicatoratnal should change.Spoking indicates a need for maintenance orthesame instantasthetransmissionofthepulse,ismostapadjustment.parentwhen in narrow rivers or closetoshoreThePPI display may appear as normal sectors alternat-The echofrom an overhead powercable appearson theing with dark sectors.This is usually due to the automaticPPI as a single echo always at right angles to the line of thefrequency control being out of adjustment.cable. If this phenomenon is not recognized, the echo can beThe appearance of serrated range rings indicates a needwrongly identified as the echo from a ship on a steady bear-formaintenance.ing.Avoiding actionresults in the echoremaining onaAfter the radar set has been turned on, the display mayconstantbearingand movingtothe samesideofthechannelnotspread immediatelytothewholeofthePPIbecauseofas the ship altering course.This phenomenon is particularlystatic electricityinsidetheCRT.Usually,the static electricapparent for the power cable spanning the Straits ofMessina.ity effect, which produces a distorted PPI display, lasts nolongerthanafewminutes.1308.AidsToRadar NavigationHour-glass effect appears as either a constriction or ex-Radar navigation aids help identifyradar targets and in-pansion of the display nearthe center of the PPI.Thecrease echo signal strengthfrom otherwise poorradar targetsexpansion effect is similar inappearancetotheexpandedcenter display.This effect, which canbe caused by a non-Buoys are particularly poor radar targets. Weak, fluc

RADAR NAVIGATION 215 of the spoke in a situation where the bearing of the Ramark sig￾nal should change. Spoking indicates a need for maintenance or adjustment. The PPI display may appear as normal sectors alternat￾ing with dark sectors. This is usually due to the automatic frequency control being out of adjustment. The appearance of serrated range rings indicates a need for maintenance. After the radar set has been turned on, the display may not spread immediately to the whole of the PPI because of static electricity inside the CRT. Usually, the static electric￾ity effect, which produces a distorted PPI display, lasts no longer than a few minutes. Hour-glass effect appears as either a constriction or ex￾pansion of the display near the center of the PPI. The expansion effect is similar in appearance to the expanded center display. This effect, which can be caused by a non￾linear time base or the sweep not starting on the indicator at the same instant as the transmission of the pulse, is most ap￾parent when in narrow rivers or close to shore. The echo from an overhead power cable appears on the PPI as a single echo always at right angles to the line of the cable. If this phenomenon is not recognized, the echo can be wrongly identified as the echo from a ship on a steady bear￾ing. Avoiding action results in the echo remaining on a constant bearing and moving to the same side of the channel as the ship altering course. This phenomenon is particularly apparent for the power cable spanning the Straits of Messina. 1308. Aids To Radar Navigation Radar navigation aids help identify radar targets and in￾crease echo signal strength from otherwise poor radar targets. Buoys are particularly poor radar targets. Weak, fluc Figure 1307b. Side-lobe effects. Figure 1307c. Multiple echoes. Figure 1307d. Second-trace echo on 12-mile range scale. Figure 1307e. Position of second-trace echo on 12-mile range scale after changing PRR

216RADAR NAVIGATIONFigure 1308a.Corner reflectors.ships within range of these beacons. There are two generaltuatingechoesreceivedfromthesetargetsareeasilylostinthe sea clutter.To aid in the detection ofthese targets,radarclasses ofthese beacons:racons, whichprovide bothbear-reflectors, designated corner reflectors,may be used.Theseingandrangeinformationtothetarget,andramarkswhichreflectors may be mounted on the tops of buoys. Additional-provide bearing information only. However, if the ramarkly,thebodyofthebuoymay be shaped as a reflector.installation is detected as an echo on the radarscope, therangewillbeavailablealsoEach cornerreflector,shown inFigure1308a,consistsof three mutuallyperpendicular flat metal surfaces.AradarA racon is a radar transponder which emits a character-wave striking any of the metal surfaces or plates will bere-istic signal when triggeredby a ship'sradar.The signal mayflectedback in thedirection of its source.Maximum energybe emitted on the same frequency as that of the triggeringwill be reflected back to the antenna if the axis of theradarradar, in which case it is superimposed on the ship's radarbeammakesequal angleswithallthemetal surfaces.Fre-display automatically.The signal maybe emitted ona sep-quently,corner reflectors are assembled in clusters toarate frequency,in which case to receive the signal themaximizethereflectedsignal.ship'sradar receivermust betuned to thebeacon frequency,Although radar reflectors are used to obtain strongeror a special receiver must be used. In either case, the PPIwill be blank except for the beacon signal.However, theechoes from radar targets, other means are requiredfor morepositive identification of radar targets. Radar beacons areonlyraconsinserviceare“inband"beaconswhichtransmitin one of the marine radar bands, usually only the 3-centi-transmitters operating in the marine radar frequency band,whichproducedistinctive indicationsontheradarscopes ofmeterband.01o0340350103502034020Cooaoo33033301o320e031031011illwa872062FtecFo0820.LeT8CKOX62OOOFowt-ofoizTeoto oo51e002091002091oit06108111061081Figure1308b.Codedraconsignal.Figure1308c.Ramark signal appearing as a broken radialline

216 RADAR NAVIGATION tuating echoes received from these targets are easily lost in the sea clutter. To aid in the detection of these targets, radar reflectors, designated corner reflectors, may be used. These reflectors may be mounted on the tops of buoys. Additional￾ly, the body of the buoy may be shaped as a reflector. Each corner reflector, shown in Figure 1308a, consists of three mutually perpendicular flat metal surfaces. A radar wave striking any of the metal surfaces or plates will be re￾flected back in the direction of its source. Maximum energy will be reflected back to the antenna if the axis of the radar beam makes equal angles with all the metal surfaces. Fre￾quently, corner reflectors are assembled in clusters to maximize the reflected signal. Although radar reflectors are used to obtain stronger echoes from radar targets, other means are required for more positive identification of radar targets. Radar beacons are transmitters operating in the marine radar frequency band, which produce distinctive indications on the radarscopes of ships within range of these beacons. There are two general classes of these beacons: racons, which provide both bear￾ing and range information to the target, and ramarks which provide bearing information only. However, if the ramark installation is detected as an echo on the radarscope, the range will be available also. A racon is a radar transponder which emits a character￾istic signal when triggered by a ship’s radar. The signal may be emitted on the same frequency as that of the triggering radar, in which case it is superimposed on the ship’s radar display automatically. The signal may be emitted on a sep￾arate frequency, in which case to receive the signal the ship’s radar receiver must be tuned to the beacon frequency, or a special receiver must be used. In either case, the PPI will be blank except for the beacon signal. However, the only racons in service are “in band” beacons which transmit in one of the marine radar bands, usually only the 3-centi￾meter band. Figure 1308a. Corner reflectors. Figure 1308b. Coded racon signal. Figure 1308c. Ramark signal appearing as a broken radial line